Consider that the vast majority of people with POTS do not eat a vegetarian/vegan diet, and also some people spontaneously recover from POTS. I know I have much less POTS-y symptoms since going vegetarian and then vegan, but the things that obviously make me worse are processed, high-glycemic foods. If you eat a completely whole food, plant-based diet with no processed foods, including minimal cooking of foods, I can't imagine that you would feel worse.
Food allergies might also play into it. It's important to be sensitive to how you react to specific foods.
